1. Field of the Invention
The present invention relates to an ignition control device for an internal combustion engine of a vehicle such as an automobile, and more particularly, to an electronic ignition control device for controlling the ignition timing of each cylinder with a computing prediction of the ignition timing based upon a ratio in timing cycles.
2. Description of the Prior Art
It is already known to control the ignition timing of an internal combustion engine of a vehicle such as an automobile by an electronic ignition control device so as to determine the timing of starting and stopping supply of electric current to an ignition coil according to an electronic computing based upon a sawtooth shaped crank position signal available from am electromagnetic pickup type rotation sensor. The sawtooth shaped signal available by such rotation sensor is generally converted into an on/off pulse signal by a comparator according to a comparison with a threshold signal level. Such a sawtooth shaped signal generally has a wave characteristic that it falls relatively steeply and rises relatively moderately in each cycle, and the time point at which the on/off pulse signal produced from such a sawtooth shaped signal turns off is highly definite but the time point at which the on/off pulse signal turns on is relatively indefinite. When the engine is operating at a relatively constant speed, it may be enough to use only the turn off information in the on/off pulse signal for controlling the ignition timing. However, when the engine is changing its rotational speed, the time span between two adjacent such turn off time points is not short enough to obtain high quality control of the ignition timing to catch up with a change of the engine speed. Therefore, it is conventional in the electronic ignition control systems to use both the turn on and the turn off information in such an on/off pulse signal for controlling the ignition timing, as in the systems disclosed, for example, in Japanese Patent Laying Open Publication Sho 57-195867 (1982), Japanese Patent Laying Open Publication Sho 60-104771 (1985), and Japanese Patent Laying Open Publication Sho 60-108565 (1985).
On the other hand, since the timings for ignition, i.e. the time points at which the supply of electric current to the ignition coil is started and stopped occur before each corresponding compression stroke of the cylinder is completed, there things need to be predicted for each ignition cycle based upon the preceding cycle times of the cylinder strokes so as to catch up with a variation of the engine rotational speed. In the conventional electronic ignition control devices a modification for each next ignition timing to catch up with a variation of the engine rotational speed is made by addition or subtraction of a certain small time difference obtained from the changes in the cycle times in the preceding engine cycles.
However, since a shifting of the turn on time point with respect to the turn off time point in the on/off pulse signal is different in each cylinder, and since the time difference to be added or subtracted changes according to the degree of acceleration or deceleration of the engine, it is difficult to predict the ignition timing to be always optimum for all cylinders.